1. Field
Implementations of the present invention relate generally to the field of photodiodes and more particularly to an avalanche photodiode (i.e., APD) with a reduced excess noise factor and an increased gain-bandwidth product.
2. Brief Description of an Illustrative Environment and Related Art
Avalanche photodiodes are incorporated in many high performance optical communications, imaging and sensing applications because they enable high signal to noise ratio and high-speed operation. An optical receiver including an APD has a signal to noise ratio that is a function of several parameters including the APD""s excess noise factor. The excess noise factor, in turn, depends on the ratio of electron and hole ionization rates in the multiplication region of the APD. The ratio of the rates at which electrons and holes impact ionize in the multiplication region is expressed in terms of the rate of impact ionization by xe2x80x9cundesirablexe2x80x9d carriers over the rate of impact ionization by xe2x80x9cdesirablexe2x80x9d carriers. For instance, if a particular APD is designed such that electrons are the charge carriers of choice, which is typical, then the impact ionization rate ratio is expressed in terms of the impact ionization rate of holes (undesirable) over the impact ionization rate of electrons (desirable). Conversely, in an APD in which holes are the carriers of choice, the impact ionization rate ratio is expressed in terms of the impact ionization rate of electrons (undesirable) over the impact ionization rate of holes (desirable). A high signal to noise ratio typically requires a low ionization rate ratio. The gain-bandwidth product also improves as the ionization rate ratio is reduced. In short, the optimization of the performance of an avalanche photodiode is largely focused on devising materials and device architecture that, when subjected to a predetermined reverse-bias voltage, result in impact ionization by a substantially greater quantity of desired carriers in one direction than undesired carriers in the opposite direction.
A typical ionization rate ratio exhibited by a silicon APD is about 0.02. However, silicon is not well suited for optical detection of wavelengths outside the range of about 0.35 to 1.0 microns. Accordingly, outside this range, other materials are typically used for detection. Alternative materials, however, are of limited utility due to less desirable impact ionization characteristics. For instance, InGaAs/InP hetero-junction APDs absorb well in the range of 1.0 to 1.65 microns, but are of limited utility because the impact ionization rate ratio is about 0.4.
Accordingly, a need exists for an avalanche photodiode with an impact ionization rate ratio better than that exhibited by other-than-silicon APDs (e.g., InGaAs/InP) and an optical sensitivity over a wavelength range exceeding that attainable with silicon APDs.
In various implementations, an avalanche photodiode structure includes a substrate, an optical absorption region and a charge-carrier multiplication region. The optical absorption region and the multiplication region are situated between a p-doped region (i.e., the anode of the APD) and an n-doped region (i.e., the cathode of the APD) that are, in order to introduce a reverse bias, placed in electrically conductive engagement with, respectively, the cathode and anode of an external energy source.
The charge-carrier multiplication region is adapted to facilitate (a) the generation, through impact ionization, of charge-carrier pairs in which each charge-carrier pair includes first and second oppositely charged charge carrier types (e.g., a negatively charged electron and a positively charged hole), and (b) the movement of charge carriers of the first type in a first direction and of charge carriers of the second type in a second direction anti-parallel to the first direction. Impact ionization by one of the first and second charge carrier types is preferred over impact ionization of the opposite charge carrier type. Typically, the preferred charge carrier type (i.e., for impact ionization) is an electron, but devices in which holes are preferred carriers are within the scope and contemplation of the invention.
Typically embodied, the charge-carrier multiplication region comprises at least one period of lattice structure comprising a first crystalline region having first and second sides and a second crystalline region having first and second ends and joined, at its first end, to the second side of the first crystalline region at a first region-second region interface. The first crystalline region is fabricated from a first material having a first impact ionization threshold and the second crystalline region is fabricated from a second material having a second impact ionization threshold. The second impact ionization threshold is lower than the first impact ionization threshold thereby rendering the second material a higher impact ionization rate material than the first material. In various embodiments, it is desirable for the interface to be as xe2x80x9cabruptxe2x80x9d as possible. That is, in such embodiments, it is desirable to achieve as clear a delineation as practicable during fabrication between the materials of the first and second regions.
Alternative embodiments include a xe2x80x9ctransitionxe2x80x9d between the first and second regions of a period that is intentionally xe2x80x9cstep-gradedxe2x80x9d or xe2x80x9ccontinuously gradedxe2x80x9d resulting in more gradual charge-carrier movement between the first and second materials. The differences between an idealized abrupt interface, a step-graded transition and a continuously graded transition may be conceptualized in terms of a selected manner of descent from a higher floor to a lower floor in a building. Crossing an ideal xe2x80x9cinterfacexe2x80x9d is analogous to freefalling to the lower floor through an elevator shaft; moving through a step-graded transition is analogous to descending a staircase and moving through a continuously graded transition is akin to descending a slide. In various embodiments, a continuously graded transition is a region exhibiting a continuous grade in energy gap and ionization threshold. A step-graded transition is a region of at least one discrete sub-region exhibiting a gap energy and ionization threshold between those of the first and second materials of the first and second regions.
A representative, non-exhaustive list of materials suitable, in various implementations, for use as the first material includes InAlAs, AIGaAs, InP, InAlGaAs, and InGaAsP. An analogous list of materials suitable for use as the second material includes InGaAs, InAlGaAs, GaAs, InGaAsP, and Si.
In various embodiments, the first and second materials are intrinsic except that the first crystalline region includes first and second oppositely doped layers separated by an intrinsic sub-region of the first crystalline region. These oppositely charged layers create a localized electric field in the sub-region between the first and second oppositely doped layers. Moreover, the magnitude of the localized electric field is elevated with respect to the xe2x80x9cbackgroundxe2x80x9d electric field resulting from the reverse-bias potential difference applied across the various regions of the APD from an external energy source.
Persons of ordinary skill in the art understand the term xe2x80x9cintrinsicxe2x80x9d to include xe2x80x9cunintentionally doped.xe2x80x9d That is, because it is extremely difficult to obtain completely xe2x80x9cpurexe2x80x9d crystal outside of regions that are intentionally doped, practitioners of the relevant art understand that, in various applications, xe2x80x9cintrinsicxe2x80x9d implies xe2x80x9cas un-doped as practicablexe2x80x9d under the design and fabrication circumstances. Accordingly, the term xe2x80x9cintrinsicxe2x80x9d as used throughout the specification and claims should not be construed so narrowly as to be limited to the nearly unattainable condition of pure, totally un-doped crystal and, in any event, should be construed at least broadly enough to include xe2x80x9cunintentionally doped.xe2x80x9d
The first and second oppositely doped layers are arranged such that each charge carrier of a set of charge carriers of the preferred type having a travel path directed toward the second end of the second crystalline region, and extending through the first region-second region interface, encounters a doped layer of the same charge prior to encountering the doped layer of the opposite charge. Accordingly, the preferred charge type is accelerated toward the second region by the localized electric field prior to passing through the first region-second region interface and entering the second region where, due to the lower impact ionization threshold of the second crystalline material, and the increased kinetic energy of the accelerated preferred charge carrier, the preferred charge carrier has a predetermined statistical probability of dissipating energy through impact ionization and generating additional pairs of first and second oppositely charged charge carrier types at a predetermined statistical rate. Conversely, although each charge carrier of a set of charge carriers of the non-preferred type that has a travel path directed across the interface toward the first end of the first region, and extending at least partially through each of the first and second crystalline materials, is accelerated by the localized electric field, that non-preferred charge carrier has a lower statistical probability of dissipating energy through impact ionization within the first material at as high a statistical rate as a charge carrier of the preferred type in the second material due to the higher impact ionization threshold of the first material.
In various embodiments including more than one period within the multiplication region, the periods are joined at periodic interfaces that are fabricated as abruptly as practicable. Alternative embodiments, however, include a xe2x80x9chole step-down regionxe2x80x9d comprising, for example, at least one discrete layer of a third material with an energy gap greater than that of the second material and lower than that of the first material and a third impact ionization threshold greater than that of the second impact ionization threshold and lower than that of the first impact ionization threshold. The purpose of such a hole step-down region is twofold: first, the layer xe2x80x9cbreaks upxe2x80x9d the energy step encountered by holes traveling from the first region of one period into the second region of a neighboring period, thereby decreasing the probability of hole-initiated impact ionization and, second, electrons traveling from the second region of one period into the first region of a neighboring period are provided with an intermediate energy step that facilitates the xe2x80x9cclimbxe2x80x9d and enhances the overall speed of the device.